When xylenes are subjected to isomerization, they form mixtures of para-xylene, ortho-xylene and meta-xylene. At the temperatures at which xylene isomerization is typically conducted, para-xylene forms approximately 24% of the equilibrium mixture, ortho-xylene about 23%, and meta-xylene about 53%. The C8 aromatic ethylbenzene is usually also present in a mixture of xylenes. Although each of these products has commercial value, the highest demand is for the para-xylene isomer.
Commercial processes for recovering para-xylene involve the selective removal of para-xylene by selective crystallization or sorption. One embodiment of a combined xylene separation process is illustrated in FIG. 1. The feed 5, having previously been stripped of all materials lighter than C8 aromatics, enters the C8 distillation column 10 and is separated into a stream 15 of C8 hydrocarbons and a stream 20 of C9+hydrocarbons. The stream 15 of C8 hydrocarbons is sent to a sorption or crystallization zone 25 where it is separated into a stream 30 of highly pure para-xylene and a stream 35 containing ortho-xylene, meta-xylene, and ethyl benzene. Stream 35 is sent to an isomerization zone 40 where the xylenes are isomerized to an equilibrium xylene mixture, and some portion of any ethylbenzene present is either dealkylated (to benzene and ethylene) or converted to a xylene. Stream 45 which is a mixture of equilibrium xylenes and unconverted ethylbenzene is then recycled to the C8 distillation column 10.
The recycle loop typically contains separation operations (not shown) downstream of the isomerization zone, including, but not limited to, one or more of a toluene splitter to remove toluene and lighter hydrocarbons from the xylenes, and a xylene splitter to remove a portion of the o-xylene and heavier hydrocarbons from the C8 hydrocarbons as a separate product. In most commercial processes, other components are present in the recycle loop, such as saturates that boil in the C8 aromatic range or other components formed in the isomerization zone, such as hydrocarbons lighter than C8, hydrocarbons heavier than C8, and naphthenes.
It would be desirable to improve the separation of xylene isomers.